Conventional pellet mills typically employ a rotating ring die with stationary rollers within a die cavity defined by the ring die. Feed material within the cavity is extruded through the openings of the ring die to form pellets. Particularly, the feed material is conventionally fed by gravity from a feed chute into the die cavity where the feed material lies in nips between the ring die and rollers within the cavity. Rotation of the ring die causes rotation of the rollers about fixed shafts within the die cavity, causing the feed material in the nips to be extruded through the ring die apertures to the outside diameter of the die, thereby forming pellets. In another form of a pellet mill such as disclosed in a companion U.S. patent application Ser. No. 09/192,436, filed Nov. 16, 1998 of common assignee herewith, the disclosure of which is incorporated herein by reference, the rollers within the die cavity are mounted on a carriage for rotation about the die cavity relative to a fixed ring die.
In both types of pellet mills, the rollers are subject to high temperatures resulting from the pelleting process. Bearing failures in the pellet mill rollers have become an increasing problem, particularly in view of demands for higher product throughput and faster continuous operation. Cooling the bearings with forced oil lubrication has not proven reliable, as shaft oil seals often fail as a result of the harsh environment of the pelleting die cavity. A grease lubricant can also be used in the bearing. However, grease does not cool the bearing or dissipate heat and, in fact, can generate heat as a result of friction when a large volume of grease is forced into the roller bearing ways. Consequently, there has developed a need for apparatus for effectively cooling the roller bearings in a pellet mill.